A high velocity layer at depth beneath Bezymyannii Volcano, a possible source of seismicity?

Volcanoes of Central Kamchatka (Klyuchevskoi, Bezymyannii, Tolbachik) are offset westward relative to the strike of the remainder of the Kamchatka volcanic arc. On the basis of local seismicity the depth of the subducting Pacific plate beneath these volcanoes is estimated to be around 150 km, while the crustal thickness estimates for Kamchatka fall into the 30-40 km range. We apply receiver function analysis to data collected by temporary broadband seismic stations installed in the vicinity of Bezymyannii Volcano, (Kamchatka, Russia) by the PIRE group in 2006 and 2007. We find evidence for a sharply defined impedance contrast in the 80-100 km depth range beneath the Klyuchevskoi group of volcanoes. The signature of this feature corresponds to an abrupt increase of seismic velocity with depth, however its depth range is not consistent with either the crust-mantle transition or the top of the subducting Pacific Plate. A review of data collected in central Kamchatka a decade ago by the Side Edge of Kamchatka Slab project suggests similar features at upper-mantle depths both south and west of the Bezymyannii. We note rare, but consistent earthquake events associated with the depth range of 75-100 km recorded in both global and local catalogs. Presence of these events within the supra-slab mantle wedge is intriguing, as it would signify a presence of brittle (cold?) material in the same depth range where we find an abrupt impedance contrast with receiver function analysis. We use data from newly available local earthquake catalogs of Kamchatka Branch of Geophysical Service to verify the accuracy of these oddly located earthquakes. We attempt to further constrain the position of these events, both relative to the main Benioff- Wadati zone seismicity and with respect to the velocity contrast we image.